The electronic parts used in the CPUs (Central Processing Units), etc. of servers, personal computers, etc. are needed to have high radiation efficiency of heat generated by semiconductor elements. For the effective radiation of the heat generated in the semiconductor elements, the structure that a heat spreader made of a material of a high specific thermal conductivity, such as copper or another, is disposed over the semiconductor element with a thermal conductive sheet, such as indium sheet or another, disposed immediately above the semiconductor element interposed therebetween.
However, due the recent large demand increase of rare metals, the price of indium has gone up, and substitute materials which are less expensive than indium are expected. In terms of the physical properties, it cannot be said that the specific thermal conductivity of indium (50 W/m·K) is high. To more effectively radiate the heat generated by the semiconductor elements, materials of higher thermal conductivities are expected.
In such background, as a material whose thermal conductivity is higher than indium, the linear structure of carbon atoms, which is typically the carbon nanotube is noted. The carbon nanotube not only has a very high thermal conductivity (1500 W/m·K) but also is superior in flexibility and heat resistance, and has high potential as a heat radiation material.
As the thermal conductive sheets using carbon nanotubes, a thermal conductive sheet including carbon nanotubes dispersed in a resin, a thermal conductive sheet including carbon nanotube bundles orientationally grown on a substrate, which are buried with a resin, etc. are proposed.
The following are examples of related art of the present invention: Japanese Laid-open Patent Publication No. 2005-150362, Japanese Laid-open Patent Publication No. 2006-147801, and Japanese Laid-open Patent Publication No. 2006-303240.
However, in the proposed thermal conductive sheets, the higher thermal conductivity of the carbon nanotube cannot be sufficiently utilized.